This invention relates to machine tools in general and in particular, to an improved spindle mechanism which makes possible a continuous change in a cutting radius, that is, the radius of revolution of the cutting tool mounted thereto. The invention is also specifically directed to a method of mounting the cutting tool to the spindle mechanism so as to derive the full benefits from its improved construction.
A change in the cutting radius is essential in some machine tools such as boring machines. A usual expedient to this end is a dual spindle assembly having a hollow outer member in which an inner member is rotatably and eccentrically mounted. The inner spindle member is adapted to eccentrically carry a cutting tool on one end thereof. A change in the relative angular positions of the outer and inner spindle members results in a change in the radius of revolution of the cutting tool about the axis of the outer spindle member. A long familiar means for causing such relative angular displacement of the nested spindle members has been differential gearing, which, however, has brought about several inconveniecnes. One of these is that the driving of the spindle assembly at two or more different speeds has been either impossible or possible only at the cost of very complex construction. Another is that the unavoidable backlashes of the differential gearing have lessened the accuracy of the relative rotary motion of the spindle members.
In order to overcome these difficulties there has more recently been suggested the use of a worm and ball nut mechanism in place of the differential gearing. It makes possible the coaxial connection of the spindle assembly to means for changing its speed of rotation. Also, being practically free from backlash, the mechanism can accurately control the relative angular displacement of the nested spindle members. Further no excessive force is exerted on the mechanism during the revolution of the spindle assembly as the latter is driven by means coupled directly to its inner member.
All these advantages have been offset, however, by one serious drawback. The worm and ball nut mechanism as heretofore suggested and used for the purpose in question has had but one worm laid parallel to the spindle assembly, and the axial motion of the ball nut on the worm has been translated into the relative angular motion of the two spindle members. As the spindle assembly is used for machining for an extended period of time, the reactive forces of the cutting operation have been easy to apply torsional stresses to the means for translating the axial motion of the ball nut into the relative angular motion of the spindle members. Such torsional stresses have eventually given rise to play between the inner and outer spindle members, thereby preventing the accurate control of the cutting radius.
Another problem encountered in the use of the spindle mechanism providing for a change in the cutting radius is the curtailment of the number of cutting tools such as boring bars used for various operations. The prior art devices have allowed, of course, the boring of various diameter holes with one and the same tool, but different tools have had to be used for different cutting operations. By the term "different cutting operations" it is meant, for instance, the cutting of aluminum, that of steel, rough machining, and fine or finish machining. All these operations require different tool materials, tip sizes and shapes, etc. Boring or like machining operations will be greatly facilitated if a single cutting tool can be used for a variety of cutting jobs.